EPISTEMIC OVERDETERMINATION AND A PRIORI JUSTIFICATION Albert Casullo University of Nebraska-Lincoln Radical empiricism is the view that experience is the only source of knowl- edge. Hence, radical empiricism denies the existence of a priori knowledge. Its most famous proponents are John Stuart Mill and W. V. Quine. Although both reject a priori knowledge, they offer different empiricist accounts of the knowl- edge alleged by their opponents to be a priori. My primary concern in this paper is not with the cogency of their positive accounts. My focus is their arguments against a priori knowledge. My goal is to establish that although they offer very different arguments against the existence of a priori knowledge, each of their arguments suffers from a common defect. They both fail to appreciate the phenomenon of epistemic overdetermination and its role in the theory of knowledge. In section 1 of the paper, I articulate Mill’s position and maintain that the key premise in his argument against the existence of a priori knowledge is a version of the Explanatory Simplicity Principle. In section 2, I elaborate the role of epistemic overdetermination in a theory of knowledge, and argue that the Explanatory Simplicity Principle is incompatible with a form of epistemic over- determination. In section 3, I turn to a version of Quine’s argument against the a priori, which has been forcefully advanced by both Hilary Putnam and Philip Kitcher, and show that the key premise of the argument is the Weak Unrevisability Condition. Finally, in section 4, I examine the relationship between epistemic overdetermination and defeasible justification, and argue that the Weak Unrevisability Condition is incompatible with a form of epistemic overdetermination. I. Mill Mill’s argument against the existence of a priori knowledge is presented within the context of offering an empiricist account of our knowledge of Philosophical Perspectives, 19, Epistemology, 2005